Hydrochemical characteristics and brine evolution paths of Lop Nor Basin, Xinjiang Province, Western China

Applied Geochemistry 25 (2010) 1770–1782

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Hydrochemical characteristics and brine evolution paths of Lop Nor Basin, Xinjiang Province, Western China Lichun Ma a,b,c, Tim K. Lowenstein c,⇑, Baoguo Li b,⇑⇑, Pingan Jiang d, Chenglin Liu a, Junpin Zhong d, Jiandong Sheng d, Honglie Qiu e, Hongqi Wu d a

Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China College of Resources and Environment, China Agricultural University, Beijing 100193, China Department of Geological Sciences, State University of New York at Binghamton, Binghamton, NY 13902, USA d College of Pratacultural and Environment Sciences, Xinjiang Agricultural University, Urumqi 830052, China e College of Natural and Social Sciences, California State University, Los Angeles, CA 90032, USA b c

a r t i c l e

i n f o

Article history: Received 9 January 2010 Accepted 12 September 2010 Available online 18 September 2010 Editorial handling by A. Vengosh

a b s t r a c t This study was carried out in the Lop Nor basin, a large arid closed drainage basin in Western China. The objective was to contribute to the understanding of the hydrological and hydrochemical processes of the Lop Nor basin by analysis of the chemical composition of different water sources and associated mineralogical characteristics of the playa sediments. The dominant river inflow waters to the Lop Nor basin are of the Na–Mg–Ca–SO4–Cl–HCO3 type. Spring inflow is dominated by Na+ and Cl. Present-day concentrated groundwater brines vary little in the study area and are consistently rich in Na+ and Cl and poor 2 2+ and SO2 in Ca2+ and HCO 3 + CO3 , but also contain a considerable amount of Mg 4 . EQL/EVP (equilibrium/evaporation), a brine equilibrium model, simulated evaporation of inflow water and groundwater brines in an open system and showed good agreement between theoretically predicted and observed minerals in the Lop Nor basin. Brine chemical modeling cannot however explain the massive amounts of glauberite (Na2SO4CaSO4) and polyhalite (K2SO4MgSO42CaSO42H2O) deposits found in a 230 m deep core ZK1200B from the Lop Nor basin. EQL/EVP simulations under a closed system allowed brine reactions with previously formed minerals and indicate that glauberite forms by back reaction between brine, gypsum and anhydrite and polyhalite forms by reaction between brine and glauberite. Diagenetic textures related to recrystallization and secondary replacement were commonly observed in core ZK1200B, indicating significant mineral–brine interaction during crystallization of glauberite and polyhalite. Mineral assemblages predicted from the evaporation of Tarim river water match closely with natural assemblages and abundances, which can explain the unusual glauberite deposits in the Lop Nor basin. It is suggested that the Tarim river inflow is the dominant source over the lake’s history. The distribution of minerals in the cored sediments documents the history of inflow water response to wet and dry periods in the Lop Nor basin. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Playas or dried lakes commonly occur in intracontinental basins with a negative water balance (Rosen, 1991; Briere, 2000; Yechieli and Wood, 2002). Whether they are called salinas, salars, salt flats, or nors, they often represent the terminus of regional surface and subsurface water flow in arid and semi-arid regions (Lines, 1979; Handford, 1982; Morrison, 1975; Rosen, 1991; Mahlknecht et al., 2004; Houston, 2006), and commonly contain a salt pan, which is seasonally wet or dry in response to changes in either regional

⇑ Corresponding author. ⇑⇑ Corresponding author. E-mail addresses: [email protected] (T.K. Lowenstein), [email protected] (B. Li). 0883-2927/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2010.09.005

recharge or evaporation (Lowenstein and Hardie, 1985; Duffy and Al-Hassan, 1988; Malek et al., 1990; Bryant, 1999; Tyler et al., 2005; Houston, 2006; Scuderi et al., 2010). Groundwater regimes play an important role in maintaining the playa system water balance, particularly in arid closed basin regions where groundwater is the dominant source (and often the only source) of water. Many studies have focused on the dynamics of the groundwater table beneath playas (Duffy and Al-Hassan, 1988; Wooding et al., 1997; Fan et al., 1997; Tyler et al., 2005), water budget, evaporative fluxes, solute transport processes, and mass equilibrium at varying time scales and have developed a series of models based on hydrology, hydrochemistry and soil physics (Duffy and Al-Hassan, 1988; Bazuhair and Wood, 1996; Tyler et al., 1997, 2005; Mason and Kipp, 1998; Tejeda et al., 2003; Risacher et al., 2003). These studies have led to a better understanding of the formation and evolution of arid basin playa systems.

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The Lop Nor is a large groundwater discharge playa located in the southeastern part of Xinjiang Province, Western China, and is also the terminal point of China’s largest endorheic drainage system, the Tarim Basin, which occupies an area of more than 530,000 km2. From a hydrochemical point of view, the Lop Nor area represents the discharge point (water and solute) of the entire Tarim drainage basin. Lop Nor is the base level of local and regional groundwater and surface water flow systems, and thus collectively captures all river and subsurface flow originating in the region over various time periods, resulting in widespread salt deposits. Present-day the Lop Nor lacks surface inflows and is characterized by desiccated saline mudflats and polygonal salt crusts. The upward capillary flux from the shallow groundwater helps to maintain a high rate of evaporation. Concentrated groundwater brine and evaporite mineral assemblages, characteristic in playa sediments, provide information about the history of water-table fluctuations, water-inflow sources and paleoclimate. The objective of this paper is to describe the principal hydrological features of the Lop Nor basin, with particular emphasis on composition and hydrochemical characterization of inflow waters and groundwater brine. Principles of brine evolution and chemical divides, theoretically calculated evolution pathways and mineral sequences, combined with stratigraphic variations in mineral assemblages from a 230 m long core in the Lop Nor basin, helped determine the primary inflow water source of the Lop Nor basin, hydrochemical evolution processes, and the history of inflow water response to regional climatic change. 2. Methods 2.1. Site description The Lop Nor basin is a large, structurally formed basin over 20,000 km2 in area, bounded by 88–92°east longitude and 39– 41°north latitude. It is located in the eastern part of the Taklamakan Desert, China’s largest and driest desert (Figs. 1 and 2). The field area (39°500 –40°400 N, 90°100 –91°300 E) for this research is

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situated in the central-eastern sector of the Lop Nor basin. That area consists of a broad, flat salt plain, with salt crusts covering approximately 5500 km2 with the lowest parts at 780 m above sea level (Zhao and Xia, 1984). The study area is remarkable in its earlike appearance as viewed by satellite imagery (Figs. 2 and 3). The Lop Nor Depression is the result of subsidence due to the collapse of the eastern Tarim Platform by tensional tectonic forces in the early Quaternary Period (Fan et al., 1987). A gravity survey verified that the Lop Nor basin is in fault contact with the Kuruktagh Mountains, Bei Shan Mountains and Altyn (Altun) Mountains (Deng, 1987). The Kuruktagh Mountains, a branch range of the Tian Shan Mountains, border the Lop Nor basin on the north. At the foot of these mountains, by the coalescence of alluvial fans, the Lop Nor rises gently to altitudes of approximately 900 m (Wang, 2001). The Bei Shan Mountains form the eastern boundary with an elevation greater than 1000 m. The Altyn (Altun) Mountains form the southern boundary, and the alluvial plain extends north of the piedmont zone at an elevation of 835–2000 m above sea level (Wang, 2001). To the west, the nearly flat desert surface slopes upward to the old, dry delta created at the mouth of the Tarim River, where it flows into the Lop Nor depression (Fig. 2). The internal morphology of the Lop Nor salt plain is shaped like a saucer with a depth of 5.2 m. The basin is asymmetrical – steeper in the SW (0.19‰) and gentler in the NE (0.09‰), although, as a whole, the surface is essentially flat and horizontal (Li et al., 2008). The surfaces of salt pans consist of pressure-ridges with welldeveloped polygonal honeycomb-shaped structures that are surrounded by a dry saline mudflat. The sediment beneath the surface is typically saturated with concentrated brines and displacive evaporites. There is visual evidence of discharging groundwater in the numerous moist salt pans covering most of the salt plain, suggesting that the capillary fringe of the groundwater table is close to the surface of the salt pans. In the wet zones, capillary action and groundwater discharge control the precipitation of evaporites and the microtopography. In these wet zones salt crusts 30–120 cm thick form on top of lacustrine sediments. Extensive accumulations of surface and subaqueous evaporites indicate that

Fig. 1. Tarim Basin, Xinjiang province, China, showing drainage system of the Basin, ancient Great-Lop-Nor region, and river flow into Lop Nor lake: KS, Kashiger River; YQ, Yerqiang River; AK, Akesu River; HT, Hetian River; KL, Keliya River; WG, Weigan River; DN, Dina River; KD, Kaidu River; CE, Cherchen River; RQ, Ruoqiang River; DR, the downstream section of the Tarim River. Modified from Ma (2007).

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Fig. 2. Modern sediments and surface geology, and the location of the ‘‘Great Ear” salt flats in the Lop Nor basin. (Background image is a Corona image mosaic, 12/12/1961.)

Fig. 3. Locations of drilling sites and transects, the spatial distribution of groundwater tables, and the investigation routes in the ‘‘Great Ear” area of Lop Nor Basin (background image is Landsat 5 TM image, 10/23/2006, pixel 30 m, R3, G2, B1).

the surface of the salt flat is still an active groundwater discharge zone of the regional drainage system. The Lop Nor salt pan is represented by a series of blackand-white concentric rings that closely resembled a human ear

in satellite images. The authors’ recent studies show that the salt crust in the ‘‘Great Ear” area exhibits lateral variations in salt content, water content, evaporite mineralogy, and micro-relief (Ma et al., 2006; Ma, 2007). High-precision elevation measurements

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by Li et al. (2008), for the first time, reveal details about the topographical characteristics of the ‘‘Great Ear”, and provide evidence to explain the formation of the ‘‘Great Ear” rings. The climate of this area is extremely arid; the average annual rainfall is less than 20 mm and the average potential evaporation rates are 3500 mm/a. The mean annual air temperature is 11.6 °C; the highest temperatures occur during July (>40 °C) and the lowest temperatures occur during January (<20° C). The primary wind direction is NE, the mean annual wind velocity is 5 m/s, and maximum velocities recorded during late spring and summer months generally exceed 14 m/s during the period of record. The Lop Nor basin experiences severe and frequent sandstorms, and is well known for its wind-eroded features, including the many meso-yardangs found along the northern, western and eastern margins of the salt plain. 2.2. Hydrologic setting As the lowest part or sink of the large Tarim Basin, Lop Nor became the terminal lake in the early Pleistocene (Fan et al., 1987). The size of the water body of ancient Great-Lop-Nor, the Lop-Sea, was once over 20,000 km2 (Wang, 2001). Shoreline features of ancient Great-Lop-Nor, such as wave-cut benches and wave-built gravel bars, can be seen on the NE boundary of the basin. The water body boundary and the origin and paths taken by rivers in historic time flowing into Lop Nor Lake are shown in Fig. 1. The Tarim River is the principal inflow river, which formed from the union of almost all the river and stream systems in this region, and ran west-to-east along the northern edge of the Taklamakan Desert, and then flowed into the Lop Nor Lake. The Tarim River has played a critical role in the hydrochemical evolution of the Lop Nor Lake. The second largest inflow is the Konche River that fed the Lop Nor from the NW. Other inflows include the Cherchen River and Ruoqiang River, at the SW flank of the Altyn Mountains, which drain into the SW part of Lop Nor Lake. However, water inflow from the Cherchen River and Ruoqiang River is minor and may be seasonal. The inflow volume response to climate change is the primary cause affecting the hydrologic balance and lake level fluctuation. Around the middle of the Pleistocene, the mountains around the Tarim Basin uplifted more than 2000–3000 m, the current prevailing wind orientation appeared, the eastern deserts of Lop Nor basin were formed (Fig. 2), and the lake water level declined (Zhao and Xia, 1984; Yuan and Yuan, 1998). During the late Pleistocene, the climate of this area became arid and cool and the Lop Nor Lake dried up (Yuan and Yuan, 1998). Climatic variation during the Holocene was dramatic, and the Lop Nor Lake underwent several significant periods of expansion and desiccation, but many details of the hydrological process and regional climate changes remain unresolved. The last record of standing water in the Lop Nor basin was described in the 1934 field investigation of Parker Chen, who reported a water body of 2400 km2 (Li et al., 2008), and 1 m deep in the center of the ‘‘Great Ear” area (Chen, 1936, 2005). The lake was fed by the Tarim and Konche Rivers from the NW. However the Lop Nor lost its main water source and dried out after the Tarim River changed course (south into Taitema Lake) following construction of a dam in 1952 (Fig. 2). Some investigators found ephemeral flooding within an area of 900 m2 near the north bank of the Lop Nor basin in the fall of 1959. The lake water salinity was 5 g/L and the depth was 50 cm (Zhou, 1978). Although the desiccated salt pan is subject to occasional flooding from the Konche River during wet periods, perennial lake conditions in the Lop Nor disappeared after 1952. Landsat satellites have provided imagery since 1972, offering insight into the Lop Nor salt pan’s changing characteristics. Since 1972 the only significant flooding observed near the mouth of the Konche River occurred in June 1973.

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At present, there is no surface water inflow to the Lop Nor playa system, and direct rainfall is negligible. Inflow from springs at the foot of the surrounding mountains and from mountain streams disappears upon reaching the alluvial fans. Small springs have been reported along the southern margin of the salt plain (Guo and Yin, 2002) (Fig. 2). Recharge from subsurface inflow from adjacent aquifers in the alluvial fans is important to maintaining the present-day playa system (Guo and Yin, 2002), but data are insufficient to quantify that recharge. Because the Lop Nor is a topographically closed basin with no surface outlet and drainage, discharge from the playa system is mainly by evaporation from the playa surface. There are a few salt tolerant plants growing along the NE and south margins of the basin, which probably transpire a small amount of water. Evaporation is the largest source of discharge on Lop Nor playa, but it is difficult to quantify because it depends on many factors such as the type and permeability of the playa surface, groundwater depth, brine density, atmospheric conditions, and solar radiation. Many techniques for estimating evaporation have been developed under different conditions and a variety of evaporation rates have been determined ranging from 10 to 230 mm/a (Allison and Barnes, 1985; Malek et al., 1990; Tyler et al., 1997). Evaporation rates of groundwater from the playa surface, estimated in the northern part of the Lop Nor basin, are 100 mm/a (Sun et al., 2008). 2.3. Field sampling and laboratory analytical techniques Field investigations and sample collection in the Lop Nor basin was done from 2005 to 2007. Groundwater was sampled with a 2-m long drill at 5-km intervals along transect O–P in September 2005, and a 1-km interval along transect B–U in September 2006 (Fig. 3). Transect O–U is 70 km and extends from the NE boundary of the Lop Nor Basin to the southwestern ‘‘ear lobe” of the ‘‘Great Ear”, and cuts through the Lop Nor basin longitudinally. Transect O–U reflects the groundwater table and chemical composition changes of the entire Lop Nor basin. Profiles were dug during 2005–2007 throughout the basin through the efflorescent crust into the underlying salt deposits and mud until brine was reached, and the sediments were sampled to define the subsurface stratigraphy and mineralogy. Springs were sampled at the southeastern edge of the basin (Fig. 3). Representative subsurface inflow water samples (50 cm below surface) were taken from the toe of the southeastern alluvial fan, 200 m south of the spring. Subsurface brines in unconsolidated salt sediments (mainly diagenetic halite) generally flowed into a brine-pool as soon as it was dug. Measurements of pH and temperature were done in situ as soon as brine was reached. When the suspended sediment had settled, samples of brine were collected into 500-mL polyethylene containers. However, in many parts of the study area, low permeability sediments occur in both the vadose and saturated zones; samples from these lower permeability lacustrine clay-rich layers were not taken. In order to examine the chemical composition of springs and groundwater, major ions were analyzed in samples that passed through 0.45 lm filters. Filtered samples were analyzed for Cl, 2+ 2+ + + SO2 4 , Ca , Mg , K and Na by Dionex DX-120 Ion Chromatography (RSD < 2%) and Perkin Elmer Optima-3300DV Inductively Coupled Plasma Optical Emission Spectrometer (RSD < 1%). Titrimetric methods (RSD < 2%) were used for alkalinity determination. A total of 18 analyses were used for the determination of the chemical composition of groundwater in the Lop Nor basin. Salt crust samples were dissolved in water and in acid for chemical analysis. Mineralogical determinations of the samples were performed using a Philips PW3040-MPD X-ray Diffractometer (XRD).

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3. Results and discussion 3.1. Spatial distribution of groundwater level The ‘‘Great Ear” area is the lowest part of the Lop Nor basin, though the entire basin is essentially a flat surface with minimal topographic relief. High rates of evaporation from the salt pan surface induce a centripetal groundwater flow pattern; therefore, the depth of groundwater beneath the ‘‘Great Ear” salt flats is a complex balance of subsurface inflow, spring inflow and high evaporation rates. Results from the 2005 to 2006 field investigation determined that the depth of groundwater varied spatially in the ‘‘Great Ear” area, but exhibited relatively stable groundwater-level trends inter-annually (Ma, 2007; Wang, 2001). Near-surface carbonate mud and crystalline gypsum and halite crusts form the shallow-brine aquifer. In the northeastern portion of the ‘‘Great Ear” area, the surface conditions are dominated by thick hard salt crusts with pressure-ridges and well-developed honeycomb polygon structures, and the subsurface consists of brine-saturated evaporite deposits (mainly halite). The groundwater in this area is generally 35–70 cm below the playa surface (Transect O–T, Fig. 3). In the central and southwestern portion of the ‘‘Great Ear” area (Transect T–U, Fig. 3), the depth to groundwater is more than 2 m. The salt pan in the southwestern area is characterized by low relief and a thin salt crust, and the subsurface sediments consist primarily of fine-grained mud, which is nearly saturated up to the contact with the salt crust. This wet salt pan zone appears darker-toned in the satellite images (see the ear lobe of the ‘‘Great Ear” in Fig. 3, the southwestern portion of Transect P–U). In the central

part of the ‘‘Great Ear” area, groundwater discharge was not observed at the surface and the salt crust is characterized by low salinity, a lack of efflorescent crusts, and significant amounts of detrital sediment. This zone appears bright-toned in the satellite images due to the higher reflectance of the dry, salt-encrusted pan surfaces (Fig. 3). 3.2. Hydrochemistry The results show that salinity and chemical composition of modern groundwater brine varies little in the ‘‘Great Ear” area (Fig. 4), and appears not to have changed significantly over the last decade (Wang, 2001; Zheng et al., 2002). The highly concentrated groundwater brine is typically rich in Na+ and Cl, poor in 2 2+ Ca2+ and HCO 3 + CO3 , contains considerable amounts of Mg , + SO2 and K , has salinities of 350 g/L (Table 1), and a pH rang4 ing from 6.6 to 7.2. The concentrated brine is saturated with respect to halite, glauberite, thenardite, polyhalite and bloedite. Groundwater brine in the northern sub-depression, called the Luobei depression (Fig. 2), is similar in chemistry and salinity to the Great Ear area (Wang, 2001), which suggests the groundwater brine in these systems is undergoing a similar chemical evolution. At present, abundant K-rich brines are stored in the subsurface crystalline glauberite deposits of the Luobei depression and have been used to produce potash fertilizer. It is estimated the Lop Nor basin will become one of the world’s largest potash-producing regions in the next few years with a potential of 3 million t/a of potash fertilizer production from solution mining (Wang, 2001).

Fig. 4. Chemical trends of the inflow waters: river (open circle)(KQ, Konche River; RQ, Ruoqiang River; CE, Cherchen River; TR1, Tarim River 1; TR2, Tarim River 2; TR3, Tarim River 3), spring (square), relatively low salinity groundwater inflow (star) and highly concentrated basin-center brine (filled circle) in Lop Nor basin. Major ion composition of all the inflow water and basin-center brine is given in Table 1.

L. Ma et al. / Applied Geochemistry 25 (2010) 1770–1782 Table 1 Major ion concentrations of inflow waters and basin center groundwater brines in the Lop Nor salt plain. All units in mg/L.

TR1 TR2 TR3 KQ RQ CE SP GW BCG1 BCG2 BCG3 BCG4

Cl

SO2 4

Ca2+

Mg2+

K+

Na+

HCO 3

157 148 243 215 241 107 2368 63,049 206,609 208,212 204,069 208,586

362 238 301 441 240 229 2038 16,704 17,429 17,594 13,206 18,170

75 91 72 53 59 34 288 978 1100 538 740 623

72 49 46 132 49 53 79 4775 12,275 18,225 10,025 25,250

8 7 8 9 10 11 32 760 8475 13,875 7575 11,725

139 119 140 173 195 103 1385 59,250 129,500 119,750 132,000 105,750

233 242 131 361 223 169 50 150 109 119 121 125

TR1, Tarim River. Reported by Fan in 1959 (Fan et al., 1987). TR2, Tarim River. Reported by Zheng et al. (2002). TR3, Tarim River. Analyzed in 2008. KQ, Konche River; RQ, Ruoqiang River; CE, Cherchen River. Reported by Fan in 1959 (Fan et al., 1987). SP, Spring inflow. GW, low salinity groundwater inflow. BCG1–BCG4, basin center groundwater brines. Analyzed in 2005–2006.

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the Tarim Basin show similar hydrochemical characteristics   (Fig. 4). The dominant anions are SO2 4 , HCO3 and Cl , and the dominant cations are Na+, Ca2+, and Mg2+ (Table 1). Surface inflows with notable SO2 4 concentrations are somewhat unusual for river water chemistry, indicating a significant interaction of river water with SO4-rich bedrock in the Tarim Basin (Wang, 2001). The Tarim River is the principal river of the Tarim Basin (Fig. 1), and it is also the longest endorheic river in China, and the fifth largest endorheic river in the world (Ma, 2007). There is considerable variation in the salinity and volume in the source area of the Tarim River, in response to seasonal and year-to-year change, but the major ion chemistry does not significantly change. The chemical analysis results of 2008 from the downstream section of the Tarim River, together with available historical data by other workers, are given in Fig. 4 and Table 1. The Tarim River is known for its high salinity in the downstream section (Fig. 1) where it reaches 1500 mg/kg, and has a pH of 7.8. This is due to long-distance water transport (2179 km), evaporative concentration, and possible recycling of surface salts. 3.3. Geochemical evolution

At the playa margins, there is groundwater discharge from the down-gradient end of the alluvial fan. These groundwater inflows have a salinity of 145 g/L, pH is 7.8, and with high concentrations of Cl, Na+, and total dissolved solids, which are possibly caused by the dissolution and recycling of halite-rich salt crusts where fresh groundwater enters the Lop Nor salt plain. The hydrochemical properties show that it is Na–Cl-rich water (Fig. 4), and 2 extremely poor in HCO 3 + CO3 . Because the concentration of 2  2 Ca2+ is greater than HCO + CO 3 3 , most of the HCO3 and CO3 will be depleted upon precipitation of CaCO3. These chemical divide characteristics also bear directly on the chemical composition of the low salinity spring water at the playa margin. The spring inflows along the playa margin have a salinity of nearly 5 g/L and a pH of 8.15, are dominated by Na+ and Cl, and are also poor in 2 HCO 3 + CO3 (Table 1; Fig. 4) due to relatively high concentrations 2+ of Ca . Although there are currently no river inflows to the Lop Nor salt plain, there are numerous rivers which flowed in the past, including the Tarim, Konche, Ruoqiang and Cherchen Rivers, which bring meteoric rain and snow melt from the Tianshan, Kunlun, and Altyn Mountains into the basin. All the inflow river waters originating in

Table 2 Mineral formulae and abbreviations discussed in text. Minerals name

Minerals codes

Mineral formulae

Anhydrite Bischofite Bloedite Calcite Carnallite Epsomite Aphthitalite Glauberite Gypsum Halite Hexahydrite Hydromagnesite Kainite Kieserite Leonite Loeweite Magnesite Polyhalite Sylvite Syngenite Tachyhydrite Thenardite

An Bis Bl Ca Car Eps Ap Gla Gy H He Hm Kai Kie Leo Lo Mg Po Syl Syn Ta Th

CaSO4 MgCl26H2O Na2SO4MgSO44H2O CaCO3 MgCl2KCl6H2O MgSO47H2O K3Na(SO4)2 NaSO4CaSO4 CaSO42H2O NaCl MgSO46H2O 3MgCO3Mg(OH)24H2O MgSO4KCl3H2O MgSO4H2O K2SO4MgSO44H2O 6Na2SO47MgSO415H2O MgCO3 K2SO4MgSO42CaSO42H2O KCl K2SO4CaSO4H2O CaCl2MgCl212H2O Na2SO4

Geochemical evolution of brine in a hydrologically closed basin is controlled primarily by the inflow composition, selective removal of solutes and evaporite precipitation (Eugster and Smith, 1965; Hardie and Eugster, 1970; Eugster and Jones, 1979; Eugster, 1980). Modeling of brine evolution pathways assumes that inflow waters (river, spring and groundwater inflow) evolve into brines which is controlled by evaporative concentration and mineral precipitation. Simulations were carried out in equilibrium mode, open system, at 25 °C, and Pco2 of 103.4 using the EQL/EVP code based on the Pitzer’s interaction model (Risacher and Clement, 2001). During evaporation, once the first (least soluble) minerals begin to precipitate, a chemical divide is created and the composition of the remaining solution changes as minerals are removed from the system. The solution then evolves along a new pathway until the next mineral reaches saturation, and a new chemical divide (turning point) is established (Hardie and Eugster, 1970; Eugster, 1980). 3.3.1. River inflow All the inflow rivers exhibit similar chemical characteristics (Fig. 4), so evaporation of any of the 4 river waters will produce similar evolution pathways and mineral precipitation sequences. Therefore, the Tarim River, the principal inflow river of the Lop Nor Basin, is used to represent all other inflow rivers. Fig. 5a presents the results of the evaporation simulation of the Tarim River water by EQL/EVP, in which two diagrams are combined: the evolution of the river water composition and the amounts of salts formed in equilibrium with the solution as it evaporates. At the beginning of the equilibration process the solution has a total salinity of 1.029 g/L (TR1, Table 1), an ionic strength of 0.024, an activity of H2O = 0.9996, and is supersaturated with respect to calcite (CaCO3). The vertical Ca2+ line in Fig. 5a indicates the removal of Ca2+ by calcite during the equilibration process. When the initial equilibration occurs (fc = 1; log = 0), the solution is equilibrated with 7.1  104 moles of calcite per kg H2O and evaporation starts. Further precipitation of calcite is limited by the low Ca2+ concentration. A total of 1.1  103 moles/kg H2O precipitates at a concentration factor of 32.56 (log = 1.51 on Fig. 5a). Hydromagnesite (3MgCO3Mg(OH)24H2O) then precipitates in relatively small amounts of 1.5  105 moles per kg H2O (fc = 37.7; log = 1.58). Next, gypsum (CaSO42H2O) forms when concentration reaches 384 times (log = 2.58) in relatively small amounts, 8.9  106 moles/kg H2O. This produces opposite evolution trends between Ca2+ and SO2 4 , and leads to a decrease in

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(a)

(b) HALITE(H) POLYHALITE(Po) KAINITE(Kai) KIESERITE(Kie) CARNALLITE(Car) BISCHOFITE(Bis)

BLOEDITE(Bl) POLYHALITE(Po) LEONITE(Leo) CARNALLITE(Car) KIESERITE(Kie) BISCHOFITE(Bis)

Fig. 5. Modeling of surface inflow water evolution pathways in the Lop Nor basin by the EQL/EVP Computer Program under open system conditions at 25 °C and Pco2 of 103.4. (a) Simulated evaporation of Tarim River water, the dominant inflow to the Lop Nor basin. (b) Simulated evaporation of spring water. The concentration factor is defined as the ratio between the initial amount of free water and the amount of water in the solution (Risacher and Clement, 2001). Mineral abbreviations and chemical formulae are shown in Table 2. In these diagrams, the top panel shows the sequence of salts precipitated, the middle panel shows the amounts of salts formed in equilibrium with the solution as it evaporates, and the bottom panel shows the evolution of the water during the evaporation process. Figs. 6–8 are the same as Fig. 5.

the concentration of Ca2+ in solution due to SO2 4 equivalents in excess of Ca2+ at the initial point of gypsum precipitation. The removal of Ca2+ results in an increasing Na:Ca ratio of the brine. As a result, glauberite, the next salt to precipitate is limited by the low Ca2+ concentration, and 1.34  105 moles/kg H2O forms from the solution (fc = 2428.3; log = 3.85). The next chemical

divide is produced by bloedite (Na2SO4MgSO44H2O) precipitation (1.6  103 moles/kg H2O), resulting in the depletion of SO2 4 due to the excess of Mg and Na at the initial point of bloedite precipitation (fc = 644; log = 2.80). Thereafter, halite (NaCl) begins precipitating (fc = 1329; log = 3.12) 2.6  103 moles per kg H2O, and the remaining brine shows an increasing Cl:Na ratio

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due to the equivalents of Cl > Na at the chemical divide created by halite. The subsequent occurrence of polyhalite (fc = 1823; log = 3.26) results in decreasing Ca2+ concentrations and increases in Mg2+ and K+ concentration; most of the Ca2+ is depleted upon precipitation of polyhalite (4.6  107 moles/kg H2O). The solution shows enrichment in Mg2+ relative to SO2 after polyhalite 4

precipitation; next, epsomite (MgSO47H2O) precipitation (fc = 2897; log = 3.46) is limited by the low SO2 concentration, and forms 4 1.2  104 moles/kg H2O. The next chemical divide point is caused by kainite (MgSO4KCl3H2O) precipitating (fc = 3466; log = 3.53), inducing an opposite trend between Cl and K+ concen trations, and also Mg2+ and SO2 4 because of excess Cl and excess

(a)

(b) BLOEDITE(Bl) BLOEDITE(Bl) POLYHALITE(Po) EPSOMITE(Eps) KAINITE(Kai)

LEONITE(Leo) SYLVITE(Syl) KAINITE(Kai) CARNALLITE(Car)

CARNALLITE(Car) BISCHOFITE(Bis)

BISCHOFITE(Bis)

Fig. 6. Modeling of groundwater inflow evolution pathways in the Lop Nor basin by the EQL/EVP Computer Program under open system conditions at 25 °C, and Pco2 of 103.4. (a) Relatively low salinity groundwater flow at the margin of the Lop Nor basin. (b) Highly concentrated basin-center brine. Mineral abbreviation codes and chemical formulas are shown in Table 2.

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Mg 2+, producing 1.6  10 4 moles/kg H2 O. The precipitation (fc = 4878; log = 3.68) of kieserite (MgSO4H2O) is also limited by the low SO2 concentration, only creating 1.1  104 moles/kg 4 H2 O, because of massive removal of SO2 from the brine by 4 early precipitation of bloedite, polyhalite and epsomite. Finally, carnallite (MgCl2KCl6H2O) and bischofite (MgCl26H2O) precipitate, forming 4.3  105 moles/kg H2O and 5  104 moles/kg H2O respectively. The final ionic strength is 12.48; activity of H2O = 0.64. Successive evaporation of river water leads to brine enrichment in Cl, and Mg2+ and a mineral precipitation sequence of increasing solubilities including calcite, gypsum, glauberite, bloedite, halite, polyhalite, epsomite, leonite, kainite, kieserite, carnallite and bischofite. Most of the salt mineral mass is contributed by halite (36%), calcite (26%), and bloedite (24%), and secondarily by bischofite (7.2%), kainite (2.3%), kieserite (2.0%) and epsomite (1.8%) (Fig. 5a). The total of other minerals is <1.2%. Combining quantitative chemical composition and semiquantitative mineralogical analysis of 76 salt crusts and short-core samples by X-ray diffraction (XRD), allows quantitative mineral composition data to be obtained from the surface evaporite deposits. These results are similar to the mineralogy observed in deep core samples by Wang (2001). Subsurface cores in the Lop Nor basin contain calcite, gypsum, halite, glauberite, polyhalite, thenardite, bloedite, loeweite, epsomite, leonite, kainite, syngenite, carnallite, bischofite and sylvite. The results indicate that evaporation modeling of river water by EQL/EVP shows good agreement between predicted and observed mineral assemblages in the Lop Nor basin. Minor variations in K and Mg salts, such as loeweite and syngenite, were observed but may be explained by evaporite diagenesis and syndepositional alteration. The only discrepancy is that thenardite, which is present in the basin, is not predicted by the EQL/EVP computer system. The absence of thenardite and the presence of a high percentage of bloedite in the paragenetic sequence suggest a Mg-rich parent brine. In addition, halite, gypsum, glauberite, bloedite and polyhalite have been reported to be the most abundant minerals in the northern Luobei depression by Liu et al. (2002, 2003). However, the predicted results show relatively less precipitation of gypsum (0.12%), glauberite (0.19%) and polyhalite (0.007%) during the direct evaporation process of river water. Secondary origins should be taken into account in these naturally deposited environments and will be discussed next.

results are shown in Fig. 6a. As demonstrated, the groundwater inflows also show an evolution pathway and mineral precipitation sequence similar to the river waters. However, the minerals precipitated from groundwater show considerable differences in relative abundance. Because the groundwater has a relatively high salinity of 145 g/L, and is depleted in Ca2+, the relatively less soluble group of minerals (calcite, gypsum) are minor. The most abundant minerals are halite (89%), bischofite (3.3%) and bloedite (2.9%). Less abundant are gypsum (1.2%), epsomite (1.0%), and kainite (1%); other minerals total less than 0.8%.

3.3.2. Spring inflow The spring water has a salinity of 6.24 g/L (SP, Table 1), an ionic strength of 0.142 and an activity of H2O = 0.997, and is supersaturated with respect to calcite. Evaporation of spring water is analogous to river waters and produces similar mineral assemblages with the exception of thenardite, leonite and epsomite (Fig. 5b). The appearance of thenardite from the evaporation of spring waters may explain its presence, though in small quantities, in the Lop Nor basin (Wang, 2001). Evaporation of spring waters does exhibit appreciable differences in relative masses of mineral constituents compared with the river waters. As indicated in Fig. 5b, the most abundant minerals precipitated from spring water are halite (76%), thenardite (12%) and gypsum (7.8%). The percentages of bloedite and bischofite are 1.8% and 0.7%, respectively, and other evaporite minerals in this system make up less than 1.3%.

The Luo Bei depression, a sub-basin of the Lop Nor basin, was cored to a depth of 230 m (Fig. 2). Wang (2001) and Liu et al. (2002, 2003) identified the mineral deposits and found clear diagenetic overprints related to dissolution, dehydration, recrystallization and secondary replacement involving glauberite and polyhalite (Fig. 9). These studies show that gypsum and anhydrite were replaced by glauberite and polyhalite. Therefore, a simple evaporative concentration model, without back reaction with previously formed salts, does not provide an adequate explanation for brine evolution and geochemical process–product relationships in the Lop Nor basin. Apart from evaporative concentration and mineral precipitation, mineral-brine reactions are important during evaporation of natural brines (Hardie and Eugster, 1970). Thus, evaporation with the EQL/EVP code was performed again under equilibrium conditions, but with a closed system, which allowed for brine reaction with previously formed minerals. Temperature and Pco2 were kept the same. In order to trace the possible mineral reactions associated with gypsum, the concentrated basin center groundwater brine (BCG1, Table 1) was diluted 5 times. The simulation results are shown in Fig. 7a. The results show that gypsum reacted with brine to form glauberite when concentrations reached 2.3–3.7 times (log = 0.34– 0.56); 0.5% glauberite by mass forming at the precipitation peak.

3.3.3. Groundwater inflow The modern Lop Nor plain is mainly fed by groundwater inflow. Therefore, the inflowing groundwater is also a significant factor in the evolution of playa brines. The groundwater at the margin of the basin has a salinity of 145 g/L (GW, Table 1), 5 times seawater, an ionic strength of 3.4 and an activity of H2O = 0.92, and is supersaturated with respect to calcite and gypsum. The simulation

3.3.4. Basin center groundwater brine Within the ‘‘Great Ear” salt pan area groundwater salinities increase to 350 g/L TDS (Table 1). Representative brines (BCG1, Table 1) have an ionic strength of 11.1 and water activity 0.56. Although some carbonate minerals such as calcite are oversaturated in the groundwater brine, they were ignored because brine alkalinity is very low (Fig. 4). The results (Fig. 6b) show that basin center groundwater brine would yield an evaporite assemblage dominated by halite (90%), bischofite (5.3%), kainite (1.8%) and carnallite (1.0%). Sylvite (0.8%), kieserite (0.5%), gypsum (0.4%) and leonite (0.2%), while glauberite (0.05%) and polyhalite (0.01%) also occur but in much smaller amounts. In summary, the predicted mineral species from simulating evaporation of inflow waters are the same as the observed minerals in the Lop Nor basin. However, as stated before, the most abundant minerals in the Lop Nor basin are halite, gypsum, bloedite, polyhalite and especially, glauberite (Wang, 2001; Liu et al., 2002, 2003). The thickness of glauberite in the subsurface varies from tens of centimeters to several meters while the relatively thin polyhalite sequence commonly is <10 cm thick (Wang, 2001). However, the evaporation results from the EQL/EVP code show that no significant quantities of glauberite and polyhalite can be obtained by direct evaporation of any inflow water sources. The predicted precipitated amounts of glauberite and polyhalite are low, varying from 0.05% to 0.23% and 0.008% to 0.01% by mass. Diagenesis could be the most widely accepted explanation of discrepancies between the observed and theoretically predicted mineral abundances. 4. Mineral composition and diagenesis information

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(a)

(b)

Fig. 7. Modeling of river inflow and basin-center brine evolution pathways in the Lop Nor basin by EQL/EVP Computer Program under closed system conditions at 25 °C, and Pco2 of 103.4. (a) Highly concentrated basin-center brine. (b) Simulated evaporation of Tarim River water. Mineral abbreviation codes and chemical formulas are shown in Table 2.

Polyhalite then replaced glauberite at fc = 3.7–40 (log = 0.56–1.6); 0.3% polyhalite by mass formed. The major salt precipitated is halite. Thus present-day brine does not favor the precipitation of large masses glauberite and polyhalite even when the role of replacement reaction is considered, because the highly concentrated Na–Cl brine, with little Ca2+, limits the precipitation of any Ca-bearing mineral. Evaporation of inflow waters (river, spring, groundwater) was also simulated by EQL/EVP code under the closed system. They should represent the ionic composition of early lake water. The results show that the simulated evaporation of the Tarim river water matches closely with natural assemblages (see Fig. 7b). At initial equilibrium,  1.1  103 moles of calcite precipitated per kg H2O as evaporation begins. Hydromagnesite then precipitates at a concentration factor of 37.6 (log = 1.57) in relatively small amounts, 1.05  106 per kg H2O. Calcite then reacts with brine to form gypsum and hydromagnesite at fc = 332 (log = 2.52), and it is completely removed at fc = 665 (log = 6.82), which corresponds to the peak of gypsum precipitation ( 1.8  103 moles/kg H2O). Simultaneously, glauberite starts precipitating at the expense of gypsum, and subsequently anhydrite forms from gypsum by dehydration, at which point a reaction

point is reached (aH2 O = 0.78) at fc = 998 (log = 3). Four minerals are in equilibrium with the solution: hydromagnesite, gypsum, anhydrite, glauberite. The evaporation process stops and all concentrations become invariant. The only reaction occurring in the system is the dehydration reaction of gypsum:

CaSO4  2H2 O ! CaSO4 þ 2H2 O The invariance ends when anhydrite completely replaces gypsum. Halite begins to precipitate at fc = 1434 (log = 3.15), which results in a decrease in Na+ and an increase in Cl because at the initial point of halite precipitation, equivalents of Cl > Na. After that, glauberite continues to precipitate at the expense of anhydrite, and reaches the precipitation peak (1.6  103 moles/kg H2O) at a concentration factor of 1542 (log = 3.18), then immediately reacts to form anhydrite and polyhalite. The appearance of polyhalite (fc = 2063; log = 3.31) leads to a noticeable decrease in concentration of K+ and Ca2+ in the solution. 1  104 moles of anhydrite and 1.6  103 moles of polyhalite per kg H2O form from the solution. Finally, bloedite and epsomite precipitate at concentration factors of 3026 (log = 3.48) and 32,479 (log = 4.5), forming 8  104 moles/kg H2O and 6.7  105 moles/kg H2O,

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respectively. The final ionic strength is 12.48, and the water activity is 0.64. The variety of minerals formed decreased under the closed system, but the most abundant minerals (halite, gypsum, glauberite, bloedite and polyhalite) in the Lop Nor basin were well predicted by the EQL/EVP code. During the evaporation process, glauberite forms by back reaction between brine, gypsum and anhydrite; polyhalite forms by reaction between brine and glauberite. Therefore, at an early stage (fc = 359–621; log = 2.5–2.8), the dominant mineral is gypsum (20–82%), and further evaporation to concentration factors between 621(log = 2.8) and 871(log = 2.9), leads to a combination of gypsum (42–82%) and glauberite (8–40%). Then glauberite (42–70%) becomes the dominant mineral with a relatively wide precipitation range (fc = 871–2063; log = 2.9–3.3). It is accompanied by gypsum (0–42%) and anhydrite (0–30%) over the first half of the stage (fc = 871–1434; log = 2.9– 3.15), then transitions to halite (7–39%) and anhydrite (6–10%) in the second half of the stage (fc = 1434–2063; log = 3.15–3.3). Finally, the evaporite assemblage is dominated by halite (39–58%) and glauberite (0–39%) with considerable amounts of anhydrite (6– 22%), and minor amounts of bloedite (0–11%) and polyhalite

(a)

(0–1.3%). This simulation compares closely with the actual mineral assemblage from the 230 m long sediment core ZK 1200B (see Fig. 9). Evaporation of spring water shows no polyhalite precipitation in place of glauberite or anhydrite (Fig. 8a). Glauberite forms by the replacement of gypsum, but it occurs as a dominant mineral (64–99%) only in a very narrow precipitation range (fc = 57–82; log = 1.76–1.91). A much narrower precipitation phase of glauberite (fc = 2.38–2.75, log = 0.37–0.44) was observed from groundwater evaporation because of early precipitation of halite due to high concentrations of Cl and Na (Fig. 8b). Only 0.5% polyhalite by mass forms at the precipitation peak (fc = 55.8–76.6; log = 1.74– 1.88). The agreement between simulated results for the Tarim River inflow and the natural mineral assemblage suggests that the Tarim River has been the dominant source over the lake’s history. Other drill cores have similar mineral assemblages (Liu et al., 2002, 2003), abundant glauberite and gypsum, and relatively small amounts of halite, polyhalite and bloedite. The distribution of minerals in the cored sediments documents the history of inflow water

(b)

THENARDITE(Th)

BLOEDITE(Bl) HEXAHYDRITE(He) BISCHOFITE(Bis)

Fig. 8. Modeling of spring water inflow and groundwater inflow evolution pathways in the Lop Nor basin by the EQL/EVP Computer Program under closed system conditions at 25 °C, and Pco2 of 103.4. (a) Simulated evaporation of spring water. (b) Relatively Low salinity groundwater flow at the margin of the Lop Nor basin. Mineral abbreviation codes and chemical formulas are shown in Table 2.

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Fig. 9. The stratigraphic variation of salt minerals and corresponding diagenetic characteristics in Core ZK 1200B. Porosity: 1. Porosity less than 5%; 2. Porosity range of 10– 15%; 3. Porosity range of 20–35%. Dissolution: 1. Near-surface dissolution; 2. Deep dissolution. Replacement: 1. Anhydrite is replaced by gypsum; 2. Anhydrite is replaced by glauberite; 3. Glauberite is replaced by polyhalite; 4. Halite is replaced by polyhalite; 5. Bloedite is replaced by polyhalite; 6. Glauberite is replaced by bloedite; 7. Halite is replaced by bloedite. Recrystallization: 1. Undeveloped; 2. Developed. Modified from Liu et al. (2002, 2003).

in response to wet and dry periods. The occurrence of abundant glauberite and gypsum below 40 m, and the absence of halite, polyhalite and bloedite in the sediment suggests that the brine has undergone incomplete evaporation (fc = 359–2063; log = 2.5– 3.3) in the wetter periods. In contrast, the increasing abundance of halite, polyhalite and bloedite in the top 40 m of core indicate relatively dry periods, because the halite precipitates at evaporative concentration of 1434 (log = 3.15), and polyhalite and bloedite precipitate at evaporative concentrations of 2063 (log = 3.31) and 3026 (log = 3.48) (Fig. 7b). Differences in inflow volume between wet and dry periods should be responsible for the vertical variation in mineral assemblages. Futhermore, the sequence of precipitating minerals is controlled by the chemical features of water prior to evaporation, and the initial composition of inflow water, which depends on regional rock weathering and water–rock interaction.

5. Conclusions The Lop Nor basin, a sub-basin of the Tarim basin, represents the discharge point of the Tarim basin, China’s largest endorheic drainage system occupying an area of more than 530,000 km2. Chemical analysis was carried out of the concentrated groundwater brines, as well as those of surface inflows and relatively low salinity groundwater inflows. Surface river inflow water compositions, obtained from historical data, suggest that rivers originating  in the Tarim Basin contain dominant anions of SO2 4 and Cl , and

cations of Na+ and Mg2+, also have a considerable amount of Ca2+ and HCO 3 . The agreement is apparent with respect to all the river inflows. Spring inflow has a salinity of nearly 5 g/L dominated by 2 the ions Na+ and Cl, and extremely poor in HCO 3 + CO3 . At the playa margins, groundwater inflows have a salinity of 145 g/L, pH is 7.8, and contain high concentrations of Cl, Na+, and total dissolved solids due to the dissolution and recycling of previous salt crusts. Modern groundwater brines in the Lop Nor basin are 2 rich in Na+ and Cl, poor in Ca2+ and HCO 3 + CO3 , but also contain a considerable amount of Mg2+ and SO2 , with salinity > 350 g/L, 4 and a pH ranging from 6.6 to 7.2. The mineral assemblage in the subsurface of the Lop Nor basin consists of calcite, gypsum, halite, glauberite, polyhalite, thenardite, bloedite, loeweite, epsomite, leonite, kainite, syngenite, carnallite, bischofite and sylvite. Evaporation modeling of inflow waters and groundwater brines by the EQL/EVP code under an open system show agreement between predicted and observed mineral species in the Lop Nor basin. Brine chemical modeling cannot however explain the massive amounts of glauberite (Na2SO4CaSO4) and polyhalite (K2SO4MgSO42CaSO42H2O) deposits found in a 230 m deep core ZK1200B from the Lop Nor basin. EQL/EVP simulations under a closed system allowed brine reactions with previously formed minerals and indicate that glauberite forms by back reaction between brine, gypsum, and anhydrite and polyhalite forms by reaction between brine and glauberite. Diagenetic overprints related to recrystallization and secondary replacement were identified in core ZK1200B, indicating a significant

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interaction of mineral-brine during crystallization of glauberite and polyhalite. Therefore a simple evaporative concentration model, without back reaction with previously formed salts, does not provide a complete explanation for brine evolution and geochemical processes-product relationships in the Lop Nor basin. Apart from evaporative concentration and mineral precipitation, mineral-brine reactions are important during evaporation of natural brines. Predicted mineral assemblages from the evaporation of the Tarim river water closely match the natural assemblages and abundances, which can explain the unusual glauberite deposits in the Lop Nor basin. This suggests that the Tarim River inflow is the dominant source and responsible for the vast quantity of salt formation over the lake’s history. The stratigraphic distribution of minerals in the cored sediments provides information on the history of water inflow in response to climate change. The general tendency towards the aridification of climate in the Lop Nor basin is shown by the change in predominant inflow water sources with time, from surface rivers to subsurface groundwater discharge. Geochemical modeling combined with stratigraphic variations in mineral assemblages in an evaporite basin can help define primary inflow waters, the history of hydrochemical evolution and regional climatic change. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 40671080), Key program of National Natural Science Foundation of China (40830420), China Postdoctoral Science Foundation funded project (Grant No. 20080430472), and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT0412). References Allison, G.B., Barnes, C.J., 1985. Estimation of evaporation from the normally ‘‘dry” Lake Frome in South Australia. J. Hydrol. 78, 229–242. Bazuhair, A.S., Wood, W.W., 1996. Chloride mass-balance method for estimating groundwater recharge in arid areas: example from western Saudi Arabia. J. Hydrol. 186, 153–159. Briere, P.R., 2000. Playa, playa lake, sabkha: proposed definitions for old terms. J. Arid Environ. 45, 1–7. Bryant, R.G., 1999. Application of AVHRR to monitoring a climatically sensitive playa. Case study: Chott el Djerid, southern Tunisia. Earth Surf. Process. Landforms 24, 283–302. Chen, Z.Q., 1936. Lop Nor Lake and Lop Nor Desert. Acta Geogr. Sin. 3, 18–49. Chen, Y.D., 2005. Go to the Refilling Lop Nor. Kunlun Press, Beijing. Deng, Z.Q., 1987. The characteristics of gravitational and magnetical field of Luobupo region in Xinjiang and its architectonic significance. Xinjiang Geol. 5, 85–91. Duffy, C.J., Al-Hassan, S., 1988. Groundwater circulation in a closed desert basin: topographic scaling and climate forcing. Water Resour. Res. 24, 1675–1688. Eugster, H.P., 1980. Geochemistry of evaporitic lacustrine deposits. Ann. Rev. Earth Planet. Sci. 8, 35–63. Eugster, H.P., Jones, B.F., 1979. Behavior of major solutes during closed-basin brine evolution. Am. J. Sci. 279, 609–631. Eugster, H.P., Smith, G.I., 1965. Mineral equilibria in the Searles Lake evaporites, California. J. Petrol. 6, 473–522. Fan, Z.L., Li, P.Q., Zhang, B.Q., 1987. The salt crust of the Lop Nur. In: Xia, X.C. (Ed.), Scientific Investigation and Research in the Lop Nur Lake Region. Science Press, Beijing, pp. 141–156. Fan, Y., Duffy, C.J., Oliver, D.S., 1997. Density-driven groundwater flow in closed desert basins: field investigations and numerical experiments. J. Hydrol. 196, 139–184.

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